1. Field of the Invention
The present invention relates to an aqueous ink, an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus.
2. Description of the Related Art
Recently, for inks used in ink jet recording apparatus, development of pigment inks with a pigment used as a coloring material have been vigorously advanced in recent years. However, pigment inks cause increase of particle size due to aggregation of pigment particles with time and sedimentation of the pigment particles according to the Stokes' law. Therefore, an ink having a high pigment concentration and containing a pigment with the particle size thereof increased (hereinafter referred to as sedimentation ink) accumulates on a bottom of an ink cartridge. As a result, in some cases, clogging in an ink flow path and at an orifice may occur to fail to achieve sufficient ejection stability.
In recent years in particular, such measures as described below have been conducted for the purpose of providing clear images little in bleeding at boundary of images formed with a black ink and color inks. In cooperation with the influences thereof, it is harder than before to retain the ejection stability of the ink. More specifically, it is conducted to use a dispersion containing carbon black high in quick drying ability as a coloring material for the purpose of forming clear images, and to make finer an ink flow path in a recording head for the purpose of forming minute ink droplets.
In order to solve the problem of the ejection stability, the following methods have been proposed. For example, it has been proposed to put balls or beads in an ink cartridge, thereby re-dispersing a sedimentation ink (see Japanese Patent Application Laid-Open No. H04-169269). An ink cartridge which is equipped with a memory means, in which data for determining the sedimentation rate of a pigment and the like in an ink has been stored, and controls an agitating operation according to the data stored in the memory means has also been proposed (see Japanese Patent Application Laid-Open No. 2003-311993).
An ink cartridge IC for which at least one of the following three ways of thinking is taken has further been proposed (see Japanese Patent No. 3,384,379).
(1) A structure with which a sedimentation ink is inhibited from being mixed into a pigment ink to be fed (structure inhibiting the feed of the sedimentation ink).
(2) A structure with which the occurrence (or precipitation) of a sedimentation ink is inhibited (structure inhibiting the occurrence of the sedimentation ink).
(3) A structure with which a sedimentation ink can be excluded at an initial feeding after preservation (structure accelerating the exclusion of the sedimentation ink).
In addition, the following proposals have been made for solving the problem of the sedimentation ink. An ink jet black ink with which a high black image density is achieved in a short drying time, and bleeding between a black image area and a color image area is little when a recording time difference between the black ink and the color ink is short has been proposed (see Japanese Patent Application Laid-Open No. 2004-10632). More specifically, the volume-average dispersed particle size of dispersed particles in an ink is from 85 nm or more to 115 nm or less, and the volume of particles having a particle size within a range of from 0.5 μm or more to 1.0 μm or less is from 0.001% or more to 0.03% or less based on the volume of the ink. In order to provide an ink excellent in long-term stability as an ink jet ink, a proposal has also been made that an ink preferably has the following parameters (see Japanese Patent Publication No. 2003-513138). More specifically, the ink has such a stability that a particle size distribution of from about 10 nm or more to 300 nm or less, and an Accusizer  number that the number of particles exceeding 0.5 μm in particle size per mL of a dispersion liquid (solid content: 15%) is less than 1010 do not change until exceeding 50% in one week at 25° C.
An investigation by the present inventors have revealed that such problems that the clogging occurs in an ink flow path and at an orifice and sufficient ejection stability is not achieved are caused even when ink cartridges satisfying the structures disclosed in Japanese Patent Application Laid-Open No. H04-169269, Japanese Patent Application Laid-Open No. 2003-311993 and Japanese Patent No. 3,384,379 are used. The reason for it is considered to be attributable to the fact that although the cartridges have an effect to slow the sedimentation phenomenon of the pigment particles in the pigment ink, the ink itself does not have a function of preventing the sedimentation of the pigment particles. As described in Japanese Patent No. 3,384,379, the structure with which a sedimentation ink in the ink cartridge can be excluded at an initial feeding after preservation is adopted, whereby the sedimentation ink can be excluded. However, it has been found that waste ink not used in recording is produced in plenty according to this method, and so the method is not preferred in this respect.
The present inventors have prepared inks satisfying the constitutions disclosed in Japanese Patent Application Laid-Open No. 2004-10632 and Japanese Patent Publication No. 2003-513138 and investigated the inks. As a result, it has been found that these inks cause clogging in an ink flow path and at an orifice, and so sufficient ejection stability is not achieved. The reason for it is considered to be attributable to the fact that although the performance for the technical level required in the prior art can be sufficiently satisfied by defining the volume and number of particles having an average particle size or particle size of 0.5 μm or more, such a technique cannot sufficiently cope with the technical level required in recent years. In other words, it is considered that the technical level required in recent years is very strict, and so such conventional techniques as described above do not come to satisfy high performance corresponding to such level.
The present inventors have thus analyzed the cause and mechanism for which the strict technical level required in recent years cannot be satisfied by using the conventional inks. As a result, the inventors arrived at the following idea. It is considered that since the number and volume of the particles having a particle size of 0.5 μm or more are in an extremely small proportion based on the whole ink, the inventions described in Japanese Patent Application Laid-Open No. 2004-10632 and Japanese Patent Publication L No. 2003-513138 only define a tiny part with respect to the whole figure of the particle size distribution.
Japanese Patent Publication No. 2003-513138 describes an ink jet ink having such a stability that the number and volume of particles having a particle size of 0.5 μm or more do not change until exceeding 50% in one week in preservation at a temperature of 25° C. However, changes in a longer period of time than this period are not specifically described, and so the ink may not be said to have sufficient performance for high-level ejection stability required in recent years.